This invention relates to an energy-efficient process and apparatus for the evaporation of water from solutions of mineral or organic materials in water, such as for example, sea water or brackish waters, hereafter sometimes referred to throughout this specification and claims as aqueous solutions.
In conventional water purification or desalination processes based on evaporation the major costs are divided approximately evenly between the cost of energy and the capital costs. Only small additional expenditures are required for operation and supplies. The energy efficiency of an evaporation process increases with decreasing temperature difference, .DELTA.T, between the condensing steam and the boiling aqueous solution. However, for practical reasons the usual water evaporation processes operate at relatively high .DELTA.T's of at least 5.degree. C. Equipment depending on nucleate boiling such as pot boiling, rising film evaporators and natural circulation evaporators requires a .DELTA.T of at least 5.degree.-8.degree. C. for efficient operation but is not used in modern desalination plants. Falling film evaporators, forced circulation evaporators, and multistage flash units are capable in principle of performing well at low temperature differences, but no equipment that is both capital efficient and energy efficient has been proposed.
In order to maintain reasonable throughputs, it is necessary when operating at low .DELTA.T's to increase the heat exchange surface areas. Since the usual evaporators are built of metal, the additional cost of such large surface elements (both materials and fabrication) is considerable.
Naturally, the additional weight and volume of such equipment also contribute to the capital costs since larger buildings are required. Finally, most metal heat exchanger elements corrode in contact with salt water and must be periodically replaced. Replacement of larger heat exchange elements costs more than replacement of smaller elements.
The decrease of the energetic efficiency of evaporators with decreasing .DELTA.T's results from the increasing importance of the energy required for pumping the aqueous solution through the system as compared with the evaporation energy saved by operating at a lower temperature difference. The following table illustrates the energy consumption and the capital investment required for conventional evaporating units operating with ocean water at a 50% conversion. The data are calculated for a vapor compression falling film evaporator.
TABLE ______________________________________ Evapora- Relative tion Pumping Total Investment Conden- Energy Energy Energy For Same sation J .times. 10.sup.6 / J .times. 10.sup.6 / J .times. 10.sup.6 / Production T .degree.C. .DELTA.T .degree.C. m.sup.3 * m.sup.3 * m.sup.3 * Rates ______________________________________ 108** 7 45 7 52 0.9 107** 6 40 8 48 1.0 106** 5 34 10 44 1.2 105 4 28 12 40 1.5 104 3 23 16 39 2.0 103 2 17 24 41 3.0 102 1 11 48 59 6.0 ______________________________________ *Based on 0.95 .times. 10.sup.6 J/m.sup.3 = 1 KWH/1000 **Normal range or prior art operation.
The above table makes it obvious why vapor compression falling film evaporators are not ordinarily operated at .DELTA.T's lower than 5.degree. C. In a falling film evaporator, the higher pumping energy requirement is due to the necessity of maintaining a continuous liquid film on the evaporating surfaces, which requires high feed rates. When the flow of liquid is inadequate, dry spots appear on heat transfer surfaces, and ultimately rivulets form, rendering most of the surface inactive. For this reason, the presently available falling film evaporators are operated at high feed rates, usually about 1000-4000 Kg per linear horizontal meter of evaporating surface, although with careful operation and excellent flow control it is possible to operate at feed rates as low as 600 kg per linear horizontal meter.
It thus is desirable to provide a less capital-intensive process and apparatus for more energetically efficient evaporation of aqueous solutions.